class Solution
{
    int m, n;
    int dx[4] = { -1, 1, 0, 0 }, dy[4] = { 0, 0, -1, 1 };
public:
    void solve(vector<vector<char>>& board)
    {
        m = board.size(), n = board[0].size();
        for (int j = 0; j < n; j++)
        {
            if (board[0][j] == 'O') dfs(board, 0, j);
            if (board[m - 1][j] == 'O') dfs(board, m - 1, j);
        }
        for (int i = 0; i < m; i++)
        {
            if (board[i][0] == 'O') dfs(board, i, 0);
            if (board[i][n - 1] == 'O') dfs(board, i, n - 1);
        }


        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (board[i][j] == 'O')
                {
                    board[i][j] = 'X';
                }
                else if (board[i][j] == '.')
                {
                    board[i][j] = 'O';
                }
            }
        }
    }

    void dfs(vector<vector<char>>& board, int i, int j)
    {
        board[i][j] = '.';
        for (int k = 0; k < 4; k++)
        {
            int x = i + dx[k], y = j + dy[k];
            if (x >= 0 && y >= 0 && x < m && y < n && board[x][y] == 'O')
            {
                dfs(board, x, y);
            }
        }
    }
};